For mutt owners, the guessing game has come to an end. Science has stepped in with the answer to the elusive question "What kind of dog is that?"

* Gallery Is this dog a pit bull?

New DNA tests, which range in cost from $55 to $200, promise to identify the breeds present in a dog's genetic soup.

Billed as a way to satisfy curiosity and to help veterinarians watch out for breed-specific ailments, the tests are also providing a dose of humility for animal specialists who now realize how bad they have been at guessing a dog's breed mix.

"I'm a veterinarian who's been classifying dogs for 10 years," said Dr. Martha Smith, director of veterinary medical services at Animal Rescue League of Boston. "And when these DNA test results started coming back, I realized that I didn't know squat."

The results of the testing have been so startling that the Animal Rescue League is planning to stop making educated guesses about mixes and will instead label all mutts as American shelter dogs. The shelter the Massachusetts Society for the Prevention of Cruelty to Animals runs at Angell Memorial Hospital is considering a similar change,although the MSPCA prefers the term New England mutt.

In most cases, attaching a label to a dog of unknown ancestry is of little importance, said Smith, who had her two dogs tested because they were getting on in years and she wanted to get to know them a little better. "In the end," she said, "a dog is a dog."Unless it's a pit bull.

Joyce Linehan's dog, Charlie, was labeled a pit bull when she adopted him from the MSPCA. Linehan, who lives in Dorchester, knew that the pit bull label meant that insurance companies, doggy day-care centers, and landlords could legally discriminate against her. But she did not know that Boston would pass an ordinance in 2004 requiring, among other things, that pit bulls and pit bull mixes be muzzled on public property.

Linehan has been a vocal opponent of the ordinance. When the DNA tests became available earlier this year, she decided to have Charlie tested, even though she was convinced that he was a pit bull.

She was wrong. The test showed he was primarily a Dalmatian-Welsh corgi mix, with only a trace of pit bull in his family tree.

The visual identification system, still standard in shelters because of the expense of the DNA test, is particularly tricky with pit bulls, Smith said.

"If a dog even winks at you like a pit bull, we have to say it's a pit bull," she said, because someone else might. If the owner is not prepared for the stigma attached to their new companion, then the dogs would be returned, she said, explaining why the Animal Rescue League errs on the side of caution.

But classifying a dog as a pit bull is a unique challenge, because a pit bull is not one breed, but a number of related ones known for broad foreheads and jaws, square muscular bodies, and short snouts. Boston's pit bull ordinance names the American Staffordshire terrier, the American pit bull terrier, the Staffordshire bull terrier, and any mixed-breed dog displaying the majority of their physical traits.

The final say on what is a pit bull and what is not falls to the city's animal control officers, who still rely on the visual method. Linehan asked the city to reclassify her dog based on the DNA test, but was told that the trace signs meant that Charlie fell under the ordinance.Rob Consalvo, the Boston city councilor who sponsored the ordinance, said the DNA testswould be a topic for a panel he is assembling to review the ordinance.

Daniel Herman was unsure whether his dog, Avey, despite her broad head, deserved her pit bull label when he and his girlfriend, Caitlin Foley, adopted Avey from the Animal Rescue League last fall. Herman knows a thing or two about genetics: He's studying for his doctorate in the subject at Harvard.

The Globe paid to have Avey's DNA tested using the $195 Wisdom Panel MX test, which requires that a blood sample be taken by a veterinarian and offers more potential breeds in its database than some of the less expensive home tests, which can be done with a cheek swab.

Foley, of Cambridge, said that while the test would not change how they looked at their dog, being able to remove the pit bull label would make life easier with insurance companies.

Herman was leaning the opposite way. He was rooting for the results to reveal that his "sweet" dog was "the most vicious combination you could imagine: the pit bull crossed with the junkyard dog."

"That would call into question how easy it is to draw conclusions based on breeds," Herman said.

The tests revealed that Avey was "an extremely complex, mixed breed dog," with traces of Chinese shar-pei, Norfolk terrier, and standard schnauzer, probably from ancestors three or four generations back, but no evidence of a pit bull breed.

"She's a good old mutt," said their veterinarian, Dr. Hugh Davis.

"I do a lot of work in southern Mexico spaying and neutering real mutty dogs," Davis said."There's a basic dog type that emerges, and she looks pretty close to it."